1. Field of the Invention
This invention relates to a novel, single stage, isolated, three-phase switching rectifier capable of high switching speeds, and more particularly, to a six-step, zero-voltage-switching rectifier, with power factor correction.
2. Description of the Prior Art
Increasing emphasis on power quality has placed a stronger demand on the performance of ac-dc converters as the front end power processing block of many electronic systems. The requirements often include:
1. unity input power factor; PA1 2. very low harmonic distortion of the input current; PA1 3. transformer isolation between source and load; and PA1 4. high efficiency and high power density.
These requirements are especially critical in high power applications, when three-phase ac-dc converters are usually employed.
Several types of three-phase ac-dc converters have been proposed which satisfy one or more of the above requirements. All of them use some form of high-frequency pulse-width-modulation (PWM) controlled rectification in order to minimize the size and weight of reactive filtering components. The first two requirements are easily met using the conventional six-step PWM technique for control of a three-phase forced-commutated bridge rectifier. Additionally, it provides maximum achievable output dc voltage without any low frequency harmonics, thus minimizing both the input and output filter requirements. Electrical isolation can be efficiently accomplished if the conversion process is divided in two stages: three-phase to high-frequency single-phase, cyclo-conversion and high-frequency ac to dc rectification. A small and efficient high-frequency transformer can then be placed between the two stages.
For an improved power density, higher switching frequencies should be used, but that would decrease efficiency due to the increased switching losses. These losses can be significantly reduced or eliminated if the zero-voltage-switching (ZVS) technique, similar to that used in the ZVS full-bridge (FB) PWM dc-dc converters as described in "Designing a High Frequency Snubberless FET Power Inverter," by Z. D. Fang et al., Proc. of POWERCON 11, D1-4, PP. 1-10, 1984, and "High-Voltage, High-Power, ZVS, Full-Bridge PWM Converter Employing an Active Snubber," by J. A. Sabate et al., IEEE APEC '91 Proc., pp. 158-163, 1991, is implemented.